web: fix --forecast breakage since 1.18 (fix #1390)

Replaces the partial HashMap.! with List.elemIndex.
(Although the HashMap is more algorithmically efficient,
its API changed recently, possibly complicating building,
and efficiency is just not going to be an issue here.)

hledger-web/Hledger/Web/Widget/Common.hs

@@ -21,12 +21,12 @@ module Hledger.Web.Widget.Common
 
 import Data.Default (def)
 import Data.Foldable (find, for_)
+import Data.List (elemIndex)
 #if !(MIN_VERSION_base(4,13,0))
 import Data.Semigroup ((<>))
 #endif
 import Data.Text (Text)
 import qualified Data.Text as T
-import qualified Data.HashMap.Strict as HashMap
 import System.FilePath (takeFileName)
 import Text.Blaze ((!), textValue)
 import qualified Text.Blaze.Html5 as H
@@ -110,12 +110,15 @@ mixedAmountAsHtml b _ =
       Just True -> "negative amount"
       _ -> "positive amount"
 
+-- Make a slug to uniquely identify this transaction
+-- in hyperlinks (as far as possible).
 transactionFragment :: Journal -> Transaction -> String
-transactionFragment j =
-    let hm = HashMap.fromList $ zip (map fst $ jfiles j) [(1::Integer) ..]
-    in  \t ->
-            printf "transaction-%d-%d"
-                (hm HashMap.! sourceFilePath (tsourcepos t)) (tindex t)
+transactionFragment j Transaction{tindex, tsourcepos} = 
+  printf "transaction-%d-%d" tfileindex tindex
+  where
+    -- the numeric index of this txn's file within all the journal files,
+    -- or 0 if this txn has no known file (eg a forecasted txn)
+    tfileindex = maybe 0 (+1) $ elemIndex (sourceFilePath tsourcepos) (journalFilePaths j)
 
 removeInacct :: Text -> [Text]
 removeInacct =